This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Variations in breast tissues resulting from the menstrual cycle are not well understood. Yet these fluctuations can compond measurements of detailed breast physiology if not properly accounted for in functional measurements. We have planned the most detailed measurements of breast tissue physiology throughout the menstrual cycle, to the best of our knowledge. Measured variations will be correlated with the expected hormonal influences that characterize menstrual cycle variations in tissues.